1.Field of the Invention
The present invention relates to vertical cavity surface emitting laser arrays and methods for manufacturing, and image forming apparatuses using the vertical cavity surface emitting laser arrays.
2.Description of the Related Art
Vertical cavity surface emitting lasers (VCSELs) having resonators formed to be perpendicular to substrate surfaces are known as one configuration of surface emitting lasers. Resonators in VCSELs are composed of two reflector mirrors disposed at opposite sides of active regions to emit beams perpendicular to the substrate surfaces.
For such surface emitting lasers, in general, distributed Bragg reflectors composed of alternately disposed layers of high and low refractive indices are used.
Extensive research and development of VCSELs has been performed since they have the following advantageous features.
Specifically, VCSELs have advantages of low threshold current and low power consumption. In addition, VCSELs have circular spots which facilitate coupling with optical devices and allow production of arrays.
Therefore, VCSELs have been expected to be applied as light sources for optical communication, optical recording, and electrophotography.
For example, for electrophotography, two-dimensionally arrayed surface emitting lasers can realize high-speed printing and high-definition image quality.
In particular, high-speed operation and high-definition images can be achieved by reducing the pitch of VCSEL devices so that many devices are densely arrayed.
However a reduced pitch of VCSEL devices results in spaces of insufficient size in order to provide wires for individually driving the two-dimensionally arrayed VCSEL devices. This makes it difficult to provide such wires.
Japanese Patent Laid-Open No. 2007-5386 discloses a surface emitting layer device with the aim of ensuring spaces of sufficient size in which wires are to be arranged for individually driving two-dimensionally and densely arrayed surface emitting laser devices in a surface emitting laser array while maintaining the size of the devices as practical.
Specifically, a surface emitting laser array disclosed in Japanese Patent Laid-Open No. 2007-5386 has a first mesa structure which includes at least a part of a first reflector mirror and does not include an active layer and a second mesa structure which includes at least an active layer. In this surface emitting laser array, the outer diameter of the second mesa structure is larger than that of the first mesa structure.
In addition, this surface emitting laser array has an electrically connected electrode on the first reflector mirror.
Specifically, as illustrated in FIG. 8, a plurality of devices are separated by forming the first and second mesa structures, and an insulating film is provided over an area except the upper surfaces of the first mesas having the electrodes provided thereon.
Further, by connecting wirings formed on the insulating film to the electrodes provided on the first mesas, the devices in the array can be individually driven.
As described above, in the surface emitting laser array disclosed in Japanese Patent Laid-Open No. 2007-5386, by providing two-stage mesa structure including the first mesa structure and the second mesa structure having different diameters, areas for wiring arrangement are ensured.
On the other hand, a surface emitting device having a wiring arranged on a separate substrate has also been proposed. The separate substrate is provided on the bottom surface of a device, which is a surface opposite to the surface where apertures are provided. Thus, there is an increased degree of freedom in arranging wires on the substrate since no restriction due to the apertures is imposed.
FIG. 9 illustrates a configuration of the surface emitting device disclosed in Japanese Patent Laid-Open No. H11-168262.
As illustrated in FIG. 9, the device has light-emitting regions (VCSELs) 915 each defining the outer diameter of an active layer, and light-emitting layers 916 constituting the active layer itself.
In this surface emitting device, a surface emitting laser array having the plural active layers 916 provided on a semiconductor substrate 901 is bonded to another substrate 912 on which wires can be arranged.
Specifically, the substrate 912 is disposed on the lower surface of the device, which is opposite the surface where apertures are provided.
In the semiconductor substrate 901, which has the apertures, portions of the semiconductor substrate 901, which is an absorber, are selectively removed to expose the apertures, and as a result the semiconductor substrate 901 is left in a frame-like shape that encloses the array area.
However, the techniques described above have the following problems.
For the surface emitting laser array disclosed in Japanese Patent Laid-Open No. 2007-5386,it is necessary to form the two-stage mesa structure and thus complicated fabrication processes are required. In addition, it is difficult to obtain a surface emitting laser having uniform characteristics.
In the surface emitting device disclosed in Japanese Patent Laid-Open No. H11-168262,the other substrate that permits wiring arrangement is disposed on the opposite side to the one where the apertures are provided. Thus, it is necessary to selectively remove portions of the semiconductor substrate, which is an absorber, since the apertures are blocked.
This results in complicated fabrication processes. In addition, it is necessary to attach the surface emitting array to the other substrate that permits wiring arrangement while taking into account the stress applied to the semiconductor substrate that has had portions thereof selectively removed to be formed into a very thin shape.
The present invention has been made in view of the above problems. Accordingly, there is a need for a vertical cavity surface emitting laser array which allows wires for individually driving devices arrayed at a small pitch to be provided on the laser array with ease and a high degree of freedom. There is also need for a method for manufacturing such a vertical cavity surface emitting laser array and an image forming apparatus using the vertical cavity surface emitting laser array.